This series of studies is aimed at understanding the effects of plasma free fatty acids (both quantity and species type) on glucose stimulated insulin secretion. We have screened 45 subjects under our screening protocol to separate subjects into those with normal vs impaired glucose and lipid tolerance and with relative low vs high insulin secretory response as assessed by the mixed meal tolerance test. The measurement of insulin secretion in these studies includes the infusion of exogenous c-peptide and measurement of c-peptide and insulin by mass spectrometry. Due to delays in obtaining an FDA approved supply of c-peptide and the installation and method development for measuring these peptides in human serum, no studies have yet been initiated beyond screening for eligible study candidates. This series of studies is aimed at understanding the normal physiology of intramyocellular lipid metabolism and its contribution to partitioning of total body fat oxidation into the adipose derived (plasma FFA) and intracellular triglyceride pools. We have screened 45 subjects under our screening protocol (#99-40) to separate subjects into those with normal vs impaired glucose and lipid tolerance and those with relative low vs high insulin secretory response as a surrogate maker for insulin sensitivity as assessed by the mixed meal tolerance test. The first two studies (#99-44 and #99-45) monitor the effect of lipid oxidation and the intramyocellular pool with prolonged fasting (36 hours) and in response to limitation of plasma FFA supply with the antilipolytic agent nicotinic acid. Studies have been hampered due to difficulties in reproducibility of lower leg placement and spacial localization of voxels for data generation. As a result preliminary studies have focused on method development using spectroscopic imaging for simultaneous multi-voxel data acquisition using subjects identified as normal controls from our screening protocol. This has been complete and manuscript is in preparation. More recent method development has focused on refining a means of fabricating custom molds for study subjects' lower legs to enable us to improve reproducibility of leg positioning. This has now reached an acceptable level of reproducibility. To date 3 subjects have been studied under protocol #99-45. Preliminary results indicate that when plasma FFA supply is blocked by administration of nicotinic acid, that the intramyocellular pool of triglycerides fall significantly indicating active oxidation of intramyocellular lipids when the supply of adipose derived fatty acids is limited. Data has not been fully analyzed for other parameters, thus these results should be considered preliminary. Study #99-45 has not yet been initiated due to persistent technical problems with the Gunhill MRI system for which this study was originally designed.